A common problem with the use of lead-acid batteries is that when the battery becomes discharged, it renders the equipment for which it is to be powered inoperative due to the lack of an adequate power source. This common problem is most evident in the use of batteries in motor vehicles since if the battery becomes discharged with time, in a discharged state it does not provide sufficient electrical energy to allow the motor vehicle to be started or the lights to be energized or the horn to be sounded. Of course, once the engine is started, the engine operates an electrical generator that is effective to charge the battery and continues to charge the battery while driving. When a motor vehicle battery is discharged to such an energy level that the motor vehicle engine cannot be started, it is common to attempt to start the engine by the use of jumper cables for connecting the discharged battery to a charged battery located in a second vehicle. This, of course, requires the second vehicle to be produced at the location of the vehicle having the discharged battery for starting the stranded motor vehicle. Commercial battery chargers such as stored in service stations are usually powered from an alternating current source and therefore require an electrical outlet.
Portable battery chargers of the type that may be readily stored in a motor vehicle so as to be transported with the motor vehicle are presently commercially available. These portable battery chargers are configured to occupy a minimum of space in the motor vehicle or in the storage compartment therefor, such as the trunk. These portable battery chargers are usually utilized to transfer sufficient charge to a discharged battery to allow a motor vehicle to be started. The presently available known portable battery chargers are powered by a number of dry cells. The advantage of the availability of the portable battery charger is that they allow the battery to be charged at the point the discharged battery is noticed without the need for a second vehicle, the use of jumper cables and/or electrical outlets.
The commercially available portable battery chargers have been found to be deficient in their implementation. These portable battery chargers depend on a direct current power pack for their power source to charge a battery. The battery pack at the time that it is required to charge a battery must have sufficient voltage and current to transfer the necessary amount of electrical current to a discharged battery in order to properly charge the battery. One such prior art unit incorporates a nickel-cadmium dry cell as the direct current power pack. A nickel-cadmium battery will transfer a high charge in a short time. However, it has been found that a major disadvantage of using such batteries is the inability of the batteries to hold a charge and therefore the user will need to constantly check to see if his portable charging apparatus is charged sufficiently for later use. Such a unit also does not incorporate any reliable method of monitoring charge transfer to the discharged battery, and the operation of the portable battery charger depends solely upon an analysis of the battery condition by the user, which can be faulty at best. Other prior art portable battery charging units have incorporated too few dry cells in their power pack to provide the desired voltage and current and at best are marginally useful for their intended purpose. These prior art units also sense the battery voltage as an indication of the charge transferred to the battery, and this technique is unreliable for such portable battery charging apparatus. Accordingly, there is a present need for an improved, more reliable, portable battery charger that can be readily stowed in a motor vehicle or on a boat that is capable of transferring sufficient energy to the battery to allow operation of a motor vehicle or to initiate the operation of the motor-generator set on a boat or the like.